1974 KoKo Sport 3000, 1987 Lotus Elite 600, 2003 Qunitana Roo Kilo tri
Craigslist for very cheap. Instant cash infusion but be prepared to loose your shirt if you really really need the money. Ebay takes fees and paypal takes days to transfer the cash to your account....then if the buyer is shady they can dispute the purchase and eBay takes the money back and "holds" it until you basically make the buyer happy. Pawn shops here won't give you hardly anything for a bike, I just don't think they sell well for them so they aren't even interested.
What babytoes said, with the emphasis on cheap. It would cost me almost $4000 to replace my favorite bike, but probably months to sell it for $3000. If I put a sign up in a bike shop asking $100, it would be gone by lunchtime. You just have to figure out what you can live with.